New and advanced continuous torque transmission systems, having continuous slip torque converters and shifting clutch systems are being developed by the automotive industry. These new systems often involve high energy requirements. Therefore, the friction materials technology must be also developed to meet the increasing energy requirements of these advanced systems.
In particular, a new high performance, durable friction material is needed. The new friction material must be able to withstand high speeds wherein surface speeds are up to about 65 m/seconds. Also, the friction material must be able to withstand high facing lining pressures up to about 1500 psi. It is also important that the friction material be useful under limited lubrication conditions.
The friction material must be durable and have high heat resistance in order to be useful in the advanced systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is being generated during operating conditions.
The high speeds generated during engagement and disengagement of the new systems mean that a friction material must be able to maintain a relatively constant friction throughout the engagement periods. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures in order to minimize “shuddering” of materials during braking or the transmission system during power shift from one gear to another. It is also important that the friction material have a desired torque curve shape so that during frictional engagement the friction material is noise or “squawk” free.
The principal performance concerns for all applications of the friction material are the prevention of shudder and the energy management of the friction interface. The occurrence of shudder can be attributed to many factors including the friction characteristics of the friction material, the mating surface's hardness and roughness, oil film retention, lubricant chemistry and interactions, clutch operating conditions, driveline assembly and hardware alignment, and driveline contamination. The friction interface energy management is primarily concerned with controlling interface temperature and is affected by the pump capacity, oil flow path and control strategy. The friction material surface design also contributes to the efficiency of interface energy management.
The main performance concerns for shifting clutch applications are the coefficient of friction characteristics of the friction material (such that the friction material has a desired torque and holding capacity) and the stability of the friction material such that the friction material does not break down under use.
Previously, asbestos fibers were included in the friction material for temperature stability. Due to health and environmental problems, asbestos is no longer being used. More recent friction materials have attempted to overcome the absence of the asbestos in the friction material by modifying impregnating paper or fiber materials with phenolic or phenolic-modified resins. These friction materials, however, do not rapidly dissipate the high heat generated, and do not have the necessary heat resistance and satisfactory high coefficient of friction performance now needed for use in the high speed systems currently being developed.
The Kearsey U.S. Pat. No. 5,585,166 describes a multi layer friction lining having a porous substrate layer (cellulose and synthetic fibers, filler and thermoset resin) and a porous friction layer (nonwoven synthetic fibers in a thermoset resin) where the friction layer has a higher porosity than the substrate layer.
The Seiz U.S. Pat. No. 5,083,650 reference involves a multi-step impregnating and curing process; i.e., a paper impregnated with a coating composition, carbon particles are placed on the paper, the coating composition in the paper is partially cured, a second coating composition is applied to the partially cured paper, and finally, both coating compositions are cured.
Various paper based fibrous materials have been developed that are co-owned by the assignee herein, BorgWarner Inc., for use in friction materials. These and all the references disclosed herein are fully incorporated herein by reference.
In particular, Lam et al., U.S. Pat. No. 5,998,307 relates to a friction material having a primary fibrous base material impregnated with a curable resin where the porous primary layer comprises at least one fibrous material and a secondary layer comprises carbon particles covering at least about 3 to about 90% of the surface of the primary layer.
The Lam et al., U.S. Pat. No. 5,858,883 relates to a base material having a primary layer of less fibrillated aramid fibers, synthetic graphite, and filler, and a secondary layer comprising carbon particles on the surface of the primary layer.
The Lam et al., U.S. Pat. No. 5,856,224 relates to a friction material comprising a base impregnated with a curable resin. The primary layer comprises less fibrillated aramid fibers, synthetic graphite and filler; the secondary layer comprises carbon particles and a retention aid.
The Lam et al. U.S. Pat. No. 5,958,507 relates to a process for producing a friction material where about 3 to about 90% of at least one surface of the fibrous material which comprises less fibrillated aramid fibers is coated with carbon particles.
The Lam, U.S. Pat. No. 6,001,750 relates to a friction material comprising a fibrous base material impregnated with a curable resin. The porous primarily layer comprises less fibrillated aramid fibers, carbon particles, carbon fibers, filler material, phenolic novoloid fibers, and optionally, cotton fibers. The secondary layer comprises carbon particles which cover the surface at about 3 to about 90% of the surface.
Yet another commonly owned patent application Ser. No. 09/707,274 relates to a paper type friction material having a porous primary fibrous base layer with friction modifying particles covering about 3 to about 90% of the surface area of the primary layer.
In addition, various paper type fibrous base materials are described in commonly owned BorgWarner Inc. Lam et al., U.S. Pat. Nos. 5,753,356 and 5,707,905 which describe base materials comprising less fibrillated aramid fibers, synthetic graphite and filler, which references are also fully incorporated herein by reference.
Another commonly owned patent, the Lam, U.S. Pat. No. 6,130,176, relates to non-metallic paper type fibrous base materials comprising less fibrillated aramid fibers, carbon fibers, carbon particles and filler.
While self-stabilizing pitch and carbon fibers made therefrom have been disclosed in various patents such as those assigned to Conaco Inc. U.S. Pat. Nos. 5,766,523; 5,540,903; 5,259,947; 5,437,780; 5,648,041; 5,501,788; 5,540,832 and 6,123,829, all of which are expressly incorporated herein by reference, none of these references teach or disclose use of a carbon fibers of pitch carbon fibers in a wet friction material application.
For all types of friction materials, in order to be useful in “wet” applications, the friction material must have a wide variety of acceptable characteristics. The friction material must have good anti-shudder characteristics; have high heat resistance and be able to dissipate heat quickly; and, have long lasting, stable and consistent frictional performance. If any of these characteristics are not met, optimum performance of the friction material is not achieved.
It is also important that a suitable impregnating resin be used in the friction material in order to form a high energy application friction material. The friction material must have good shear strength during use when the friction material is infused with brake fluid or transmission oil during use.
Accordingly, it is an object of the present invention to provide an improved friction material with reliable and improved properties compared to those of the prior art.
A further object of this invention is to provide friction materials with improved “anti-shudder”, “hot spot” resistance, high heat resistance, high friction stability and durability, and strength.